1. Field of the Invention
The present invention relates to a drift detection method for a control apparatus controlling an injection molding machine, which method detects drift in a pressure detection value output from a pressure detector such as a load cell.
2. Description of the Relevant Art
As disclosed in Japanese Patent Publication (kokoku) No. 7(1995)-67720, in general, a control apparatus for controlling an injection molding machine includes a pressure control system which controls pressure such as injection pressure or back pressure, through feedback control, on the basis of a preset pressure command value and a pressure detection value output from a pressure detector such as a load cell. Such a control apparatus performs zero-point adjustment for the pressure detection value output from the pressure detector (load cell), in order to compensate for variation in the characteristics of the pressure detector itself and a mechanical error present at a portion where the pressure detector is attached. However, since the zero point set through an initial adjustment frequently drifts due to, for example, change in the external environment or long-term variation, in general, zero-point adjustment (re-adjustment) for eliminating drift is performed regularly or as needed.
An example method for performing such zero-point adjustment is disclosed in Japanese Patent No. 2657352. In this method, a pressure detection value is obtained in a predetermined period during operation of an injection molding machine in which the pressure detection value theoretically becomes zero, such as a predetermined period after completion of initialization performed when power is activated or a predetermined period after completion of an injection step; and correction is performed on the basis of the thus-obtained pressure detection value.
However, the above-described conventional method has the following drawbacks to be solved.
First, in the conventional method, an absolute zero point is detected in order to correct for drift on the basis of the zero point. However, drift cannot be detected accurately and stably, because the zero point tends to be influenced by disturbances such as variation in position of a screw.
Second, since detection and correction are performed in a predetermined period during operation of an injection molding machine in which the pressure detection value theoretically becomes zero, the conventional method is restricted with respect to the period (time) for performing correction, thereby lowering the degree of flexibility in performing detection processing and correction processing.